This application relates generally to non-woven fabrics produced from low modulus polymeric materials and to a method for their manufacture.
More specifically, this invention relates to non-woven, needle bonded fabrics manufactured from fibers having a unique combination of physical properties.
Historically, man-made fibers have been engineered so that the physical properties of such fibers are about the same as textile fibers found in nature, for example, cotton or wool. Natural textile fibers are generally thin, having a diameter less than about 2 mils and having a high elastic modulus, for example, a modulus greater than about 200,000 psi. Thus, synthetic fibers are thin and have a high modulus. For example, a typical commercially-available, polyethylene monofilament having a tensile strength of about 28,500 psi displays an elastic modulus of about 340,000 psi. Such thin, high modulus fibers have a stiffness parameter generally ranging between about 1.times.10.sup.-5 and about 1.times.10.sup.-8 lb-in.sup.2. In general, any fiber having a stiffness parameter within this range will feel soft and pliant. Because conventional fibers have a relatively high elastic modulus, usually well above 200,000 psi, they must have a relatively low moment of inertia, otherwise they would feel too stiff.
Elastic modulus, designated as E.sub.f, is determined by measuring the initial slope of the stress-strain curve derived according to ASTM standard method No. D2256-69. Strain measurements are corrected for gauge length variations by the method described in an article entitled "A Method for Determining Tensile Strains and Elastic Modulus of Metallic Filaments," ASM Transactions Quarterly, Vol. 60, No. 4, December 1967, pp. 726-27.
The moment of inertia, designated I.sub.f, of a fiber is a function of its cross-sectional area. Under normal loading conditions, fibers bend about a neutral axis where the moment of inertia will be a minimum value. The moment of inertia about this neutral axis is calculated using the following equation: EQU I.sub.f =.intg.y.sup.2 dA
Where dA is any incremental area of the fiber's cross-section and y is the distance any such incremental area is from the neutral axis.
For fibers with a uniform circular cross-sectional configuration, the moment of inertia (I.sub.f) may be calculated by the following formula: EQU I.sub.f =.pi.d.sup.4 /64
where d is the fiber diameter. Specific equations for calculating the moments of inertia of fibers having a cross-sectional configuration other than circular are given in a paper presented at the 47th annual meeting of the ASTM, Vol. 44, (1944).
The stiffness parameter of a fiber, designated K.sub.f, is a general indicator of the feel, or hand, of a fabric made from that fiber. When considering the hand of any fiber, one must take into account the specific textile construction in which the hand is being judged. In a non-woven needle punched fabric of light weight, for example, the feel of the fabric depends upon the texture, flexibility and overall softness of the fabric.
Somewhat different, but related, criteria apply to heavier, non-woven fabrics used as carpeting. In this application, one primary consideration is the walking comfort or "underfoot bounce" of the carpet. This property is related to the ability of the carpet to absorb energy and give a comfortable feel underfoot.
Another consideration of importance in carpet fabrics is the appearance of the floor covering after use. This property is related to the fabric's ability to recover from compressive forces which in turn depends heavily on the energy absorbed in compression. Although some of the energy can be recovered in the form of viscoelastic work, much of it is lost and is manifested as a permanent deformation of the pile. The effect of abrasion coupled with the inability of a fabric to recover from deformation is commonly referred to as matting which manifests itself as a deterioration in the visual appearance of the carpet.
Although other factors affect the hand of pile fabrics, the chief factor is the fiber stiffness which is a function of the material properties of the fiber, the geometry of the fiber and the manner in which load is applied to the fiber. In general terms, one may compare the hand of different fabrics by comparing the stiffness parameter of the fibers, where each fiber has a uniform cross-section and is composed of the same material throughout. This stiffness parameter is the product of the elastic modulus of the fiber and the area moment of inertia of the fiber: EQU K.sub.f =E.sub.f .times.I.sub.f